Stellar Nucleosynthesis


Stellar nucleosynthesis is the process by which elements are created within stars by combining the protons and neutrons together from the nuclei of lighter elements. This is not the only form of nucleosynthesis, however, but usually when scientists talk about nucleosynthesis, it's shorthand for stellar nucleosynthesis.

History of the Theory

The idea that stars fuse together the atoms of light elements was first proposed in the 1920's, by Einstein's strong supporter Arthur Eddington. However, the real credit for developing it into a coherent theory is given to Fred Hoyle's work in the aftermath of World War II. Hoyle's theory contained some significant differences from the current theory, most notably that he did not believe in the big bang theory but believed instead that hydrogen was continually being created within our universe. (This alternative theory was called a steady state theory and fell out of favor when the cosmic microwave background radiation was detected.)

The Early Stars

The simplest type of atom in the universe is a hydrogen atom, which contains a single proton in the nucleus (possibly with some neutrons hanging out, as well) with electrons circling that nucleus. These protons are now believed to have formed when the incredibly high energy quark-gluon plasma of the very early universe lost enough energy that quarks began bonding together to form protons (and other hadrons, like neutrons). Hydrogen formed pretty much instantly and even helium (with nuclei containing 2 protons) formed in relatively short order (part of a process referred to as Big Bang nucleosynthesis).

As this hydrogen and helium began to form in the early universe, there were some areas where it was more dense than in others. Gravity took over and eventually these atoms were pulled together into massive clouds gas in the vastness of space. Once these clouds got large enough they were drawn together by gravity with enough force to actually cause the atomic nuclei to fuse together, in a process called nuclear fusion. The result of this fusion process is that the two one-proton atoms have now formed a single two-proton atom. In other words, two hydrogen atoms have begun one single helium atom. The energy released during this process is what causes the sun (or any other star, for that matter) to burn.

It takes nearly 10 million years to burn through the helium and then things heat up and the helium begins fusing together. Stellar nucleosynthesis continues to create heavier and heavier elements, until you end up with iron ... but I'll leave it for the Krauss quote below to give you the elegant details of this process.

Lawrence Krauss on Stellar Nucleosynthesis

Here are a couple of great quotes on the subject from a 2011 talk by astrophysicist Lawrence Krauss. (Here's a link to the video.) They really describe the process of nucleosynthesis in a far more beautiful manner than I can:

“Every atom in your body came from a star that exploded. And, the atoms in your left hand probably came from a different star than your right hand. It really is the most poetic thing I know about physics: You are all stardust. You couldn’t be here if stars hadn’t exploded, because the elements - the carbon, nitrogen, oxygen, iron, all the things that matter for evolution and for life - weren’t created at the beginning of time. They were created in the nuclear furnaces of stars, and the only way for them to get into your body is if those stars were kind enough to explode.... The stars died so that you could be here today.”

"All the hydrogen burns into helium in 10 million years.... All the helium burns to carbon in 1 million years.... Again, the star starts to collapse, because there's no more fuel. But then it heats up and the carbon starts to burn ... to form neon and nitrogen. And all of the carbon in the star burns in 100 thousand years.... And you get to oxygen. Oxygen ... burns to silicon in 10 thousand years. It's getting hotter and hotter and hotter. Less efficient. And then when all the oxygen burns to silicon, you're in the last day of the star because, remarkably, it is so hot at that point that all of the silicon in the center of the star, many thousands of times the mass of the Earth, burns to form iron in one day.... Iron can't burn to form anything. Iron is the most tightly-bound nucleus in nature. So once that's happened, there's no more fuel... When all the silicon has burned to iron, suddenly the star realizes there's no place left to go and that interior of the star, which has been held up by the pressure of nuclear burning, collapses. That whole collapse happens in one second.... There's a shock wave and that shock wave ... spews out all of the atoms that were created during the life history of a star. The carbon, the nitrogen, the helium, the iron. And that's vitally important, because every atom in your body was once inside a star that exploded.... The atoms in your left hand probably came from a different star than in your right hand, because 200 million stars have exploded to make up the atoms in your body."

Also Known As: nucleosynthesis